The present invention relates generally to the field of microelectronics, and more specifically to methods for fabricating a package having an integral window, for packaging at least one microelectronic device.
Many different types of microelectronic devices require a window to provide optical access and protection from the environment. Examples of optically-interactive semiconductor devices include charge-coupled devices (CCD), photo-sensitive cells (photocells), solid-state imaging devices, and UV-light sensitive Erasable Programmable Read-Only Memory (EPROM) chips. All of these devices use microelectronic devices that are sensitive to light over a range of wavelengths, including ultraviolet, infrared, and visible. Other types of semiconductor photonic devices emit light, such as laser diodes and Vertical Cavity Surface-Emitting Laser (VCSELS), which also need to pass light through a protective window.
Microelectromechanical systems (MEMS) and Integrated MEMS (IMEMS) devices (e.g. MEMS devices combined with Integrated Circuit (IC) devices) can also require a window for optical access. Examples of MEMS devices include airbag accelerometers, microengines, microlocks, optical switches, tiltable mirrors, miniature gyroscopes, sensors, and actuators. All of these MEMS devices use active mechanical and/or optical elements. Some examples of active MEMS structures include gears, hinges, levers, slides, tilting mirrors, and optical sensors. These active structures must be free to move or rotate. Optical access through a window is required for MEMS devices that have mirrors and optical elements. Optical access to non-optically active MEMS devices can also be required for inspection, observation, and performance characterization of the moving elements.
Additionally, radiation detectors which detect alpha, beta, and gamma radiation, use xe2x80x9cwindowsxe2x80x9d of varying thickness and materials to either transmit, block, or filter these energetic particles. These devices also have a need for windows that transmit or filter radiation to and from the device, while at the same time providing physical and environmental protection.
The word xe2x80x9ctransparentxe2x80x9d is broadly defined herein to include transmission of radiation (e.g. photons and energetic particles) covering a wide range of wavelengths and energies, not just UV, IR, and visible light. Likewise, the word xe2x80x9cwindowxe2x80x9d is broadly defined herein to include materials other than optically transparent glass, ceramic, or plastic, such as thin sheets of metal, which can transmit energetic particles (e.g. alpha, beta, gamma, and light or heavy ions).
There is a continuing need in the semiconductor fabrication industry to reduce costs and improve reliability by reducing the number of fabrication steps, while increasing the density of components. One approach is to shrink the size of packaging. Another is to combine as many steps into one by integrating operations. A good example is the use of cofired multilayer ceramic packages. Unfortunately, adding windows to these packages typically increases the complexity and costs.
Hermetically sealed packages are used to satisfy more demanding environmental requirements, such as for military and space applications. The schematic shown in FIG. 1 illustrates a conventional ceramic package for a MEMS device, a CCD chip, or other optically active microelectronic device. The device or chip is die-attached face-up to a ceramic package and then wirebonded to interconnect inside of the package. Metallized circuit traces carry the electrical signal through the ceramic package to electrical leads mounted outside. A glass window is attached as the last step with a frit glass or solder seal. Examples of conventional ceramic packages include Ceramic Dual In-Line Package (CERDIP), EPROM and Ceramic Flatpack designs.
Although stronger, ceramic packages are typically heavier, bulkier, and more expensive to fabricate than plastic molded packages. Problems with using wirebonding include the fragility of very thin wires; wire sweep detachment and breakage during transfer molding; additional space required to accommodate the arched wire shape and toolpath motion of the wirebond toolhead; and the constraint that the window (or cover lid) be attached after the wirebonding step. Also, attachment of the window as the last step can limit the temperature of bonding the window to the package.
FIG. 2 illustrates schematically a conventional molded plastic (e.g. encapsulated) microelectronic package. The chip or device is attached to a lead frame, and a polymer dam prevents the plastic encapsulant from flowing onto the light-sensitive area of the chip or device during plastic transfer molding. The window is generally attached with a polymer adhesive. Problems with this approach include the use of fragile wirebonded interconnections; and plastic encapsulation, which does not provide hermetic sealing against moisture intrusion.
Flip-chip mounting of microelectronic devices is a commonly used alternative to wirebonding. In flip-chip mounting the chip or device is mounted face-down and then reflow soldered using small solder balls or xe2x80x9cbumpsxe2x80x9d to a substrate having a matching pattern of circuit traces (such as a printed wiring board). All of the solder joints are made simultaneously. Excess spreading of the molten solder ball is prevented by the use of specially-designed bonding pads. Flip-chip mounting has been successfully used in fabricating Multi-Chip Modules (MCM""s), Chip-on-Board, Silicon-on-Silicon, and Ball Grid Array packaging designs.
Flip-chip mounting has many benefits over traditional wirebonding, including increased packaging density, lower lead inductance, shorter circuit traces, thinner package height, no thin wires to break, and simultaneous mechanical die-attach and electrical circuit interconnection. Another advantage is that the chips are naturally self-aligning due to surface tension when using molten solder balls. It is also possible to replace the metallic solder bumps with bumps, or dollops, of an electrically-conductive polymer or epoxy (e.g. silver-filled epoxy). Flip-chip mounting avoids potential problems associated with ultrasonic bonding techniques that can impart stressful vibrations to a fragile (e.g. released) MEMS structure.
Despite the well-known advantages of flip-chip mounting, this technique has not been widely practiced for packaging of MEMS devices, Integrated MEMS (IMEMS), or CCD chips because attaching the chip face-down to a solid, opaque substrate prevents optical access to the optically-active, light-sensitive surface.
The cost of fabricating ceramic packages can be reduced by using cofired ceramic multilayer packages. Multilayer packages are presently used in many product categories, including leadless chip carriers, pin-grid arrays (PGA""s), side-brazed dual-in-line packages (DIP""s), flatpacks, and leaded chip carriers. Depending on the application, 5-40 layers of dielectric layers can be used, each having printed signal traces, ground planes, and power planes. Each signal layer can be connected to adjacent layers above and below by conductive vias passing through the dielectric layers.
Electrically conducting metallized traces, thick-film resistors, and solder-filled vias or Z-interconnects are conventionally made by thick-film metallization techniques, including screen-printing. Multiple layers are printed, vias-created, stacked, collated, and registered. The layers are then joined together (e.g. laminated) by a process of burnout, followed by firing at elevated temperatures. Burnout at 350-600 C. first removes the organic binders and plasticizers from the substrate layers and conductor/resistor pastes. After burnout, these parts are fired at much higher temperatures, which sinters and densifies the glass-ceramic substrate to form a dense and rigid insulating structure. Glass-forming constituents in the layers can flow and fill-in voids, corners, etc.
Two different cofired ceramic systems are conventionally used, depending on the choice of materials: high-temperature cofired ceramic (HTCC), and low-temperature cofired ceramic (LTCC). HTCC systems typically use alumina substrates; are printed with molybdenum-manganese or tungsten conducting traces; and are fired at high temperatures, from 1300 to 1800 C. LTCC systems use a wide variety of glass-ceramic substrates; are printed with Au, Ag, or Cu metallizations; and are fired at lower temperatures, from 600 C. to 1300 C. After firing, the semiconductor die is attached to the fired HTCC (or LTCC) body; followed by wirebonding. Finally, the package is lidded and sealed by attaching a metallic, ceramic, or glass cover lid with a braze, a frit glass, or a solder seal, depending on the hierarchy of thermal processing and on performance specifications.
Use of cofired multilayer ceramic structures for semiconductor packages advantageously permits a wide choice of geometrical designs and processing conditions, as compared to previous use of bulk ceramic pieces (which typically had to be cut and ground from solid blocks or bars). Ceramic packages with high-temperature seals are generally stronger and have improved hermeticity, compared to plastic encapsulated packages. It is well known to those skilled in the art that damaging moisture can penetrate polymer-based seals over time. Also, metallized conductive traces are more durable than freestanding wire bond segments, especially when the traces are embedded and protected within a layer of insulating material.
In summary, conventional methods and designs for packaging of light-sensitive microelectronic devices attach the window (or cover lid containing a window) after completing the steps of die attachment and wirebonding of the chip or MEMS device to the package. Many processing steps are used, which can expose the fragile MEMS structures to particulate contamination and mechanical damage during packaging.
What is needed is a packaging process that minimizes the number of times that a MEMS device is handled and exposed to temperature cycles and different environments, which can possibly lead to contamination of the device. This can be accomplished by performing as many of the package fabrication steps as possible before mounting the MEMS device. What is needed, then, is a packaging process that generally attaches the window to the package before mounting the chip or device to the package. It is also desired that the window be attached to the package body at a high temperature to provide a strong, hermetic bond between the window and the body. What also is needed is a method where the MEMS device faces away from the cover lid, so that contamination is reduced when the cover lid is attached last. A process is needed that provides for releasing MEMS structures after the MEMS device has been mounted to the package, but before attaching the window.
Electrical interconnections from the chip or device to the package are needed that are stronger and less fragile than conventional wirebonds. What also is needed is a package having a high degree of strength and hermeticity. In some cases, it is also desired to stack back-to-back multiple chips, of different types (e.g. CMOS, MEMS, etc.) inside of a single, windowed-package on multiple levels.
Use of the phrase xe2x80x9cMEMS devicesxe2x80x9d is broadly defined herein to include xe2x80x9cIMEMSxe2x80x9d devices, unless specifically stated otherwise. The word xe2x80x9cplasticxe2x80x9d is broadly defined herein to include any type of flowable, dielectric composition, including polymer compounds and spin-on glass-polymer compositions. The phrases xe2x80x9creleased MEMS structuresxe2x80x9d, xe2x80x9creleased MEMS elementsxe2x80x9d, and xe2x80x9cactive MEMS elementsxe2x80x9d and xe2x80x9cactive MEMS structuresxe2x80x9d are used interchangeably to refer to a device having freely-movable structural elements, such as gears, pivots, hinges, sliders, tilting mirrors; and also to exposed active elements such as chemical sensors, flexible membranes, and beams with thin-film strain gauges, which are used in accelerometers and pressure sensors.